Staphylococcus aureus is the second leading cause of hospital-acquired pneumonia and an important agent of severe community-acquired pneumonia. Pneumonia is both an important preceding event to acute lung injury and the most commonly fatal complication of ARDS. We hypothesize that early host:pathogen interactions in the lung determine whether S. aureus establishes a successful infection or is eliminated by innate defenses. There is a limited understanding of the pathogenesis of pneumonia, especially the initial microbial:host interaction. Using state of the art in vivo model systems and genomic/proteomic technology we will investigate the early microbial and host responses in the first few hours after bacterial entry into the lungs. Four aims will address our hypothesis. In aim 1 we will characterize changes in bacterial gene expression using genomic array analysis in response to the lung environment during the first few hours of infection. Aim 2 will examine the host proteins in airway fluid, which bind to the bacterial surface and correlate these to changes in gene expression examined over the same time frame as observed in aim 1. In addition, we will characterize the mouse airway fluid proteome, which has yet to be characterized. These studies may identify the host proteins which serve as signals to the organism during the initial phases of infection and further define those which serve as innate defenses against infection for the host. Aim 3 will determine the role of Toll-Like Receptors (TLR) on the interaction of S. aureus with the host, using animals deficient in TLR expression; specifically defective for expressing the downstream signaling molecule MyD88. These studies will evaluate the importance of the initial inflammatory response to bacterial survival and host immunity. The final aim will extend the above experiments by further examining the importance of the bacterial surface associated protein genes with antisense gene inhibition technology in our animal model of pneumonia, to identify those genes and their products which are primarily involved in the initial interactions in the airway. These studies will yield novel insights into bacterial pathogenesis in the lower respiratory tract by examining both sides of the host:pathogen interface in the early stages of infection. These insights could have important implications for the mechanisms underlying the initiation of pneumonia, acute lung injury, and its sequelae.